Internal combustion engines may include water injection systems that inject water into a plurality of locations, such as into an intake manifold, upstream of engine cylinders, or directly into engine cylinders. Engine water injection provides various benefits such as an increase fuel economy and engine performance, as well as a decrease in engine emissions. In particular, when water is injected into the engine intake or cylinders, heat is transferred from the intake air and/or engine components to evaporate the water, leading to charge cooling. Injecting water into the intake air (e.g., in the intake manifold) lowers both the intake air temperature and a temperature of combustion at the engine cylinders. By cooling the intake air charge, a knock tendency may be decreased without enriching the combustion air-fuel ratio. This may also allow for a higher compression ratio, advanced ignition timing, improved wide-open throttle performance, and decreased exhaust temperature. As a result, fuel efficiency is increased. Additionally, greater volumetric efficiency may lead to increased torque. Furthermore, lowered combustion temperature with water injection may reduce NOx emissions, while a more efficient fuel mixture (less enrichment) may reduce carbon monoxide and hydrocarbon emissions.
Engine control systems may select when to use water injection based on engine operating conditions, such as engine knock limitations. One example approach is shown by Surnilla et al. in U.S. Pat. No. 8,096,283. Therein, water usage is based on water availability, knock limits, dilution requirements, and spark constraints. Another example approach is shown by Connor in U.S. Pat. No. 5,148,776. Therein water usage is adjusted based on the amount of cooling required to overcome premature ignition of an air-fuel mixture in engine cylinders.
However the inventors herein have recognized potential issues with such approaches. As one example, for water injection into an engine having a step-ratio transmission, the optimal fuel economy gain associated with water usage may not be realized due to the fixed gear ratio of the transmission. In particular, at a given driver demand, based on whether water is being injected or not, there may be an associated fixed engine speed and load range that meets the driver demand. An engine controller may use water injection based on water availability on-board the vehicle. However, when transitioning between operating with or without water injection, there may be engine limitations experienced at the associated engine speed-load that may reduce the fuel economy benefit of the transition. As an example, when water injection is not being used, the engine may become more knock-limited at high loads. Consequently, the optimum engine speed-load for the driver demand may be different from that when water injection is used. Another issue is that frequent changes in operator pedal demand may cause the engine load to move back and forth, leading to frequent switching on and off of water injection. Excessive switches can degrade fuel economy due to losses incurred during transitions, and may degrade the life of the parts. In addition, the frequent switching can result in speed/load and air/fuel ratio disturbances. The issue may be exacerbated in a hybrid vehicle where the engine encounters multiple engine pull-ups and pull-downs (such as during frequent start/stop events) wherein the engine is restarted or shutdown while the vehicle is being propelled.
The inventors herein have recognized that the operating efficiency of a hybrid powertrain may be improved (e.g., maximized) by determining the most efficient water injection state at the driver demanded power, while compensating with battery power, and additionally while smoothing torque transients using motor torque. In particular, battery power can be leveraged to reduce the frequency of water injection state switching while also improving the operating efficiency, without being hindered by associated constraints and trade-offs. In addition, the fuel economy benefits of an engine configured with water injection may be better leveraged through integration with a hybrid transaxle (such as a modular hybrid transmission, or MHT) which may enable the engine speed and load to be adjusted based on water usage (and availability) while maintaining the power output of the engine. In one example, fuel economy may be improved by a method for a hybrid vehicle including an engine configured with water injection and a modular hybrid transaxle (MHT). The method may comprise: for a power level, comparing a first fuel economy without water injection and a first amount of stored power offset from an energy storage system to a second fuel economy with water injection at a first adjusted engine speed-load and a second amount of stored power offset; responsive to the second fuel economy exceeding the first fuel economy, and a higher than threshold water availability, injecting an amount of water into the engine and changing to the first adjusted engine speed-load; and responsive to the first fuel economy exceeding the second fuel economy or a lower than threshold water availability, operating the engine without water injection, and changing the engine speed-load to a second adjusted engine speed-load.
As an example, a hybrid vehicle system may be configured with a battery powered electric motor for propelling vehicle wheels via motor torque, an engine configured with water injection, and a hybrid transaxle (such as an MHT). Water may be injected from a water reservoir into an intake manifold of the engine via one or more of central and port injection, and/or directly into an engine cylinder. At any given driver demand, the controller may be configured to compare the fuel efficiency versus power with and without water injection. The controller may further calculate the efficiency of each water injection state with a range of battery offsets, where the energy efficiency of the electrical system to generate, store, discharge, and propel is combined with the energy efficiency of the engine to determine a total efficiency for each possible battery offset. The battery offsets may be determined based on the state of charge of the system battery and may include a positive offset (wherein battery power via battery discharging is used to boost engine output) as well as a negative offset (wherein battery power via battery charging is used to adjust engine output). The controller may then select whether to continue in the current water injection state (with or without battery offset) or transition to the water injection state (with or without battery offset) by comparing the corresponding fuel efficiencies. Specifically, if a higher than threshold improvement in efficiency is achieved by transitioning to the other water injection state, the transition may be performed, else the current water injection state may be maintained. In addition, the battery offset corresponding to the more efficient state may be applied. Any transients incurred during the transition may be smoothened using motor torque. Also following the selection of the more efficient water injection state, the controller may use motor torque adjustments as well as hybrid transaxle adjustments to operate the engine in a narrow speed-load operating range where efficiency of the selected water injection state is optimized, while maintaining a given power level of the vehicle. For example, to address knock anticipated while operating without water injection, an engine controller may select a gear ratio of the MHT to increase the engine speed while decreasing the engine load so as to maintain the demanded engine power output. Likewise, when operating with water injection active, a gear ratio of the MHT may be selected to lower the engine speed (relative to the previous engine speed when water injection was inactive) while load is increased (as compared to the previous load when water injection was inactive). Because the quantity of water is limited in the reservoir, a vehicle controller may aim to use the water only when a pre-determined improvement in fuel efficiency occurs, so it only injects the water and adjusts the speed-load when the “water” efficiency improvement exceeds a threshold over the non-water speed-load efficiency.
In this way, fuel economy benefits can be improved. The technical effect of integrating water injection technology in a vehicle having an MHT transmission is that for a given driver demanded power, the benefits of the water injection can be better leveraged. In particular, the engine speed and torque for a given driver demanded power can be adjusted to reduce knock limitations at higher loads to increase the maximum load, and reduce friction losses at lower loads, while accounting for changes in knock limits due to water injection properties. One of the technical effects of using battery power to extend operation of the engine with a given water injection state is that losses associated with frequent switching of the water injection state are reduced. In particular, battery power can be used to keep operating the engine on a current water injection state at a more efficient power. While operating the engine with the more efficient water injection state, battery power can be used up to a threshold to make up any difference in output. While operating the engine with the more efficient and cost-effective water injection state, MHT adjustments can be used to extend engine operation with water injection despite changes in driver or wheel torque demand, and for conditions where the benefit of water injection is small, MHT adjustments can be used to extend engine operation without water injection despite changes in driver or wheel torque demand. By optimizing water usage, the benefits of water injection can be extended over a longer portion of a drive cycle, even when water availability is limited. In this way, an engine can be operated with water injection while providing an improved fuel economy for a given driver demand by increasing the maximum load that can be achieved without knocking, or in other words, by increasing the knock limit.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.